The present invention relates to an antenna device in which a plurality of element antennas is arranged, for example, in a communication or radar so as to form a beam.
FIG. 12 is a diagram showing a conventional antenna device which is disclosed in, for example, Japanese Patent Laid-Open No. 7-288417. Referring to FIG. 12, reference numeral 1 denotes element antennas which are arranged on a plane, and reference numeral 2 is concentric circles along which the plurality of element antennas 1 are arranged. Each of the element antennas 1 is connected with a feed means that adjusts an excitation amplitude or an excitation phase.
Then, the operation of the above-mentioned conventional antenna device will be described. The excitation amplitude and the excitation phase of each of the element antennas 1 are adjusted by the feed means, so that the antenna device of the present invention is capable of obtaining a desired radiation characteristic.
Also, FIG. 13 is a diagram showing another conventional antenna device which is disclosed in, for example, 1999 IEEE, AP-S, pp. 2032-2035, xe2x80x9cDesign of low side lobe circular ring arrays by element radius optimizationxe2x80x9d. The figure shows the arrangement of the element antennas of an array antenna in which the element antennas 1 are arranged along the concentric circles 2. Here, reference numeral 4 denotes coordinates.
Referring to FIG. 13, a table indicative of intervals of the concentric circles represents the intervals of the concentric circles 2 by a wavelength unit. In the table, a right column shows a case in which the respective concentric circles 2 are arranged at regular intervals, whereas a left column shows a case in which the intervals of the concentric circles 2 are so adjusted as to reduce a side lobe.
Then, the operation of another conventional antenna device will be described. In the conventional antenna device, the side lobe is reduced by adjustment of the intervals of the concentric circles 2. The adjusting manner is that a desired radiation pattern is regulated, and the radius of each of the concentric circles 2 is determined sequentially from the inner side so as to approximate the desired radiation pattern.
Here, in order to avoid a quarter grating lobe stated below, the intervals of the respective concentric circles 2 are limited to one wavelength or shorter. Note that, the above document discloses that the side lobe level of a portion in the vicinity of a main beam, which is xe2x88x9217.7 dB in the case where the intervals of the concentric circles are equal to each other is reduced to xe2x88x9227.4 dB in the case where the intervals of the concentric circles are adjusted.
In the array antenna, it is general that the arrangement of the element antennas is of a rectangular arrangement or a triangular arrangement from the viewpoint of easiness in structuring a feed system or the like. In the rectangular arrangement or the triangular arrangement, when the intervals of the element antennas (hereinafter referred to as xe2x80x9celement intervalsxe2x80x9d) are widened in order to reduce the number of element antennas, the grating lobe having substantially the same level as that of the main lobe occurs, resulting in a problem such as the radiation in an unnecessary direction, or the like. On the contrary, in the concentric circle arrangement described in the above-mentioned conventional example, there is advantageous in that a definite grating lobe does not occur even if the element intervals are widened.
However, even in the concentric circle arrangement, when the element intervals are widened, a side lobe having a level of some degree which should be regarded as a quarter grating lobe over a wide angle occurs, with the result that there may arise a problem from the viewpoint of the unnecessary radiation suppression.
FIG. 11(a) shows one example. FIG. 11(a) is a diagram showing the radiation pattern (radiation characteristic) of an array antenna in which 18 concentric circles are arranged at regular intervals. The element antennas 1 are arranged relatively thickly on a circumference of each of the concentric circles 2 to prevent a high side lobe from occurring due to the widened element intervals in the circumferential direction. Also, the element intervals are equal to each other along the circumferential direction of all the concentric circles 2, and all of the element antennas 1 are equal to each other in amplitude.
An abscissa axis u of FIG. 11(a) represents a u-coordinate (which will be described in the description of the embodiments) which corresponds to a wave-number space, and a main beam is structured when u=0. When the intervals of the concentric circles 2 are widened, a visible region where the radiation pattern appears in a real space is widened. For example, in the case where the main beam is along a crest direction which is perpendicular to an antenna plane, the region of 0xe2x89xa6uxe2x89xa66.28 becomes the radiation pattern of the real space when the intervals of the concentric circles 2 are 1xcex (xcex is a wavelength), and the region of 0xe2x89xa6uxe2x89xa612.57 becomes the radiation pattern of the real space when the intervals of the concentric circles 2 are 2xcex.
As is understood from FIG. 11(a), when the intervals of the concentric circles 2 become larger than about 1xcex, the side lobe of xe2x88x9220 dB level which is relatively large appears over the wide angle. The appearance of the side lobe depends on the intervals of the concentric circles 2, and in the case where the main beam is scanned over the wide angle, the side lobe appears in the real space even when the intervals of the concentric circles 2 are smaller than 1xcex. The wide angle side lobe level hardly changes even if the number of concentric circles 2 increases, and is about xe2x88x9220 dB in the case where an amplitude distribution of an opening is uniform.
As described above, in the conventional regular-interval concentric circle arrangement, there arises such a problem that the side lobe which is high in the level over the wide angle occurs when the intervals of the concentric circles 2 increase for the purpose of reducing the number of element antennas 1 or the like.
Also, in the case where the intervals of the concentric circles 2 are narrow, there is shown a manner in which the side lobe is reduced by adjusting the intervals of the concentric circles 2 as described in the other conventional antenna device. However, in the case where the intervals of the concentric circles 2 are 1xcex or more, there is no proposal of the effective manner.
The present invention has been made in order to solve the above-mentioned problems, and therefore an object of the present invention is to obtain an antenna device which is capable of suppressing an unnecessary side lobe over the wide angle in the case where intervals of concentric circles are widened.
According to claim 1 of the present invention, there is provided an antenna device, including a plurality of concentric circle array antennas each having a different radius on an identical plane, in which a plurality of element antennas are arranged circumferentially in each of the concentric circle array antennas, in which the plurality of concentric circle array antennas are arranged at regular intervals d in most part thereof, and in which the concentric circle array antennas corresponding to a remaining part of the plurality of concentric circle array antennas are arranged at intervals dxc2x1(0.4 to 0.6)d.
According to claim 2 of the present invention, in the antenna device according to claim 1 of the invention, the interval of the plurality of concentric circle array antennas is set to one wavelength or longer.
According to claim 3 of the present invention, there is provided an antenna device, including a plurality of concentric circle array antennas each having a different radius on an identical plane, in which a plurality of element antennas are arranged circumferentially in each of the concentric circle array antennas, in which the plurality of concentric circle array antennas are divided into groups including four continuous concentric circle array antennas, and one of the four concentric circle array antennas which are included in each of the groups is arranged at an interval dxc2x1(0.4 to 0.6)d, and in which the three remaining concentric circle array antennas in each of the groups are arranged at the regular intervals d.
According to claim 4 of the present invention, in the antenna device according to claim 3 of the invention, the interval of the plurality of concentric circle array antennas is set to one wavelength or longer.
According to claim 5 of the present invention, there is provided an antenna device, including: a first concentric circle array antenna having a plurality of element antennas arranged at regular intervals in a circumferential direction and having a radius an=Lnxc2x7d where a radius coefficient is Ln (n is an integer), and a reference interval of the concentric circle array antennas is d; and a second concentric circle array antenna having a plurality of element antennas arranged at regular intervals in a circumferential direction and having a radius an+1=Ln+1xc2x7dxc2x1(0.4 to 0.6)d.
According to claim 6 of the present invention, in the antenna device according to claim 5 of the invention, the interval of the first and second concentric circle array antennas is set to one wavelength or longer.